iterator-pattern

🧠 Concept

The Iterator Pattern provides a way to access elements of a collection sequentially without exposing the collection’s internal structure.

👉 It separates traversal logic from the collection itself, allowing flexible iteration across different types of collections.

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🧱 Key Roles

Role	Description
Iterator	Interface defining traversal methods (hasNext(), next()).
ConcreteIterator	Implements the actual iteration logic.
Aggregate (Collection)	Interface defining a method to create an iterator.
ConcreteAggregate	Implements the collection and provides its iterator.

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💻 Java Example – Report Iterator

// Iterator interface
interface Iterator<T> {
    boolean hasNext();
    T next();
}

// Aggregate interface
interface ReportCollection {
    Iterator<String> createIterator();
}

// Concrete collection
class ReportList implements ReportCollection {
    private List<String> reports = new ArrayList<>();

    void addReport(String report) {
        reports.add(report);
    }

    public Iterator<String> createIterator() {
        return new ReportIterator(reports);
    }
}

// Concrete iterator
class ReportIterator implements Iterator<String> {
    private List<String> reports;
    private int index = 0;

    ReportIterator(List<String> reports) {
        this.reports = reports;
    }

    public boolean hasNext() {
        return index < reports.size();
    }

    public String next() {
        return hasNext() ? reports.get(index++) : null;
    }
}

✅ Usage Example:

ReportList reportList = new ReportList();
reportList.addReport("Sales Report");
reportList.addReport("Employee Report");
reportList.addReport("Inventory Report");

Iterator<String> iterator = reportList.createIterator();

while (iterator.hasNext()) {
    System.out.println(iterator.next());
}

/*
Sales Report
Employee Report
Inventory Report
*/

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🧠 Flow Summary (Step-by-Step)

Step	Action	Iterator State	Output
1️⃣	createIterator()	index = 0	—
2️⃣	hasNext() → true	index = 0	—
3️⃣	next()	index = 1	"Sales Report"
4️⃣	next()	index = 2	"Employee Report"
5️⃣	next()	index = 3	"Inventory Report"
6️⃣	hasNext() → false	—	Iteration ends

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🪜 Summary

• Provides a uniform interface to traverse different collections.

• Encapsulates iteration logic inside a separate object.

• Allows multiple iterators on the same collection.

• Promotes Single Responsibility and Open/Closed Principles.

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📊 Real-world Analogy

📚 Book Shelf Iterator – You can go through books one by one using the same method (next()), without knowing how they’re stored (array, list, or database).